The present invention relates generally to the field of organometallic compounds. In particular, the present invention relates to monoalkyl Group VA metal compounds which are suitable for use as intermediates in the preparation of precursors for chemical vapor deposition.
Metal films may be deposited on surfaces, such as non-conductive surfaces, by a variety of means such as chemical vapor deposition (“CVD”), physical vapor deposition (“PVD”), and other epitaxial techniques such as liquid phase epitaxy (“LPE”), molecular beam epitaxy (“MBE”), and chemical beam epitaxy (“CBE”). Chemical vapor deposition processes, such as metalorganic chemical vapor deposition (“MOCVD”), deposit a metal layer by decomposing organometallic precursor compounds at elevated temperatures, i.e. above room temperature, either at atmospheric pressure or at reduced pressures.
A wide variety of metals may be deposited using such CVD or MOCVD processes. See, for example, Stringfellow, Organometallic Vapor Phase Epitaxy: Theory and Practice, Academic Press, 2nd Edition, 1999, for an overview of such processes. Organometallic compounds of arsenic, antimony, and bismuth are used to deposit epitaxial films in the semiconductor and related electronic industries. Epitaxial films such as gallium arsenide find applications in optoelectronic devices such as detectors, solar cells, light-emitting diodes (“LED's”), lasers and electronic switching devices such as field effect transistors (“FET's”) and high electron mobility FET's (“HEMT's”). Ternary arsenic alloys also exist such as gallium indium arsenide (“GaInAs”) and aluminum indium arsenide (“AlInAs”), which are more attractive than GaAs or aluminum gallium arsenide (“AlGaAs”) for the most powerful fiber optic systems operating in the 1.3 to 1.55 micron wavelength range. Gallium arsenide phosphide (“GaAsP”) is suitable for visible LED's and fiber optic emitters/detectors. Antimony and antimony alloy films are useful in fiber optics communication systems, particularly in the 1.3 and 1.55-micron regions. Antimony-containing semiconductor materials also have commercial applications including detection for seeker, night vision and surveillance devices (infrared detectors) and sources (LED's or lasers). A variety of binary, ternary and quaternary Group III/V semiconductor systems containing antimony have been evaluated for applications in infrared emitters and detectors operating in the 3 to 5 micron and 8 to 12 micron spectral ranges. These wavelength ranges are important since they are natural windows in the atmosphere for infrared transmission. Epitaxial antimony-based Group III/V semiconductors have potential applications in long wavelength detectors and high-speed electronic devices.
Arsine (“AsH3”) and phosphine (“PH3”) are attractive precursors for MOVPE since they provide arsenic and phosphorus along with hydrogen radicals that can scavenge any carbon-containing radicals generated during the MOVPE growth. However, the highly toxic nature of arsine and phosphine makes handling these gases in cylinders at high pressures dangerous. The threat of their rapid release in large quantities is serious and significantly high facility costs are often incurred to meet the appropriate safety requirements. Thus, there is a need to develop alternative Group VA hydride precursor compounds that are less hazardous than arsine and phosphine. Certain trialkyl Group VA metal compounds, such as trialkyl stibines, have been developed. However, such trialkyl compounds typically have low vapor pressures and higher decomposition temperatures. Such trialkyl compounds also result in carbon incorporation in the grown films. Monoalkyl Group VA dihydride compounds are excellent alternatives as they greatly reduce the amount of carbon incorporated in grown metal films.
For semiconductor and electronic device applications, these Group VA metal alkyls must be highly pure and be substantially free of detectable levels of both metallic impurities, such as silicon and zinc, as well as oxygenated impurities. Oxygenated impurities are typically present from the solvents used to prepare such organometallic compounds, and are also present from other adventitious sources of moisture or oxygen.
One method of preparing monoalkyl arsines and phosphines reacts arsine or phosphine gas with an alkene in the presence of a catalyst. See, for example, European Patent No. EP 579 248 B1 and European Patent Application No EP 560 029 A1. Another method reacts arsine with metallic sodium in liquid ammonia followed by reaction with an alkyl halide. See Magihara et al., Handbook of Organometallic Compounds, W. A. Benjamin, Inc., New York, 1968, pp 560, 566, 571, 574, and 579-580. Both of these approaches require the handling of arsine or phosphine, which are both very toxic.
Grignard type syntheses are also known. For example, arsenic trihalide or a phosphorus trihalide is reacted with an alkyl Grignard reagent to form monoalkyl arsenic or phosphorus dihalides which are subsequently reduced to form monoalkyl arsine or phosphine. See, for example, Japanese Patent Application No. JP 10-130 288. Such reactions are carried out in ethereal solvents. Other preparation methods utilizing ethereal solvents are known. See, for example, Japanese Patent Application No. JP 07-285977. The monoalkyl arsines and phosphines produced by these methods require extensive purification in order to remove the ethereal solvent. Even with such purification procedures, trace ethereal solvents remain in the monoalkyl arsines and phosphines.
Aluminum alkyls can have been used as reagents in the preparation of Group VA metal trialkyl compounds. For example, Zakharkin et al., Bull. Acad. Sci. USSR, 1959, p1853, discloses a method of producing trialkyl compounds of antimony and bismuth, as shown in equation (I), where R is ethyl, n-propyl or iso-butyl and X is chloride or fluoride.MX3+R3Al+diethylether→MR3+AlX3   (I) 
Trace amounts of ethereal solvent invariably remain in the target organometallic compound obtained using conventional techniques. Such residual ethereal solvent contributes oxygen as a deleterious impurity in metal films deposited from such precursor compounds.
Attempts have been made to synthesize trialkyl Group VA organometallics in non-ethereal solvents. For example, Takashi et al., J. Organometal. Chem., 8, pp 209-223, 1967, disclose the reaction of antimony trichloride with triethylaluminum in hexane. Such reaction was found to produce triethylstibine in extremely low yields (only about 10%), the remainder being about 42% metallic antimony and about 46% of an antimony-aluminum complex, (SbEt4)(Al2Et5Cl2). This article does not teach how to obtain triethylstibines free of antimony-aluminum complexes.
These trialkyl aluminum reaction approaches have been attempted only in the preparation of certain Group VA metal trialkyl compounds. Such approach has not been disclosed in the preparation of Group VA metal monoalkyl compounds.
Accordingly, there is a need for methods for preparing Group VA metal monoalkyls in high yields and for Group VA metal compounds substantially free of both metallic and oxygenated impurities for use as precursor compounds for CVD.